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Published online 11 August 2008 | Nature | doi:10.1038/news.2008.1033
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Reversing the prism
A slab of material that bends light the wrong way could herald true invisibility.
Materials that could one day make objects invisible to visible light have been devised by scientists at the University of California at Berkeley1,2.
These artificial substances, called metamaterials, have a negative refractive index, so that light reflected from or passing through the metamaterial is bent the wrong way.
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It is not clear what precisely is meant by invisibility. A material object is truly invisible if it is perfectly transparent to visible light and does not at all modify its propagation, so that light from the background reaches an observer exactly in a manner that is unaltered by the object. So far we know that in practice an object is visible: (1) Either because it is self-luminous that causes its being distinguished from the background. Or (2) it partially or totally reflects light so that the reflection of light from the foreground makes its presence felt. Or (3) it is partially or completely opaque to light, either due to absorption of the latter or scattering thereof. Then a fall in the intensity of light from the background makes its presence obvious. Or (4) it is perfectly transparent but has an index of refraction different from that of the surrounding medium. That would then lead to a distorted vision of the background over a patch that corresponds to the objects outline. There are situations when an object could become invisible. For example, a self luminous object against a matching luminous background appears invisible. A blackbody is invisible against a dark background. For an object covered with metamaterial to appear invisible it would be necessary that light bends around it and finally leaves along its original path of propagation. That would mean that a light beam remembers its history and would know at which 'point' on the other side of the object it has to leave the object and in which 'direction' it has to continue its propagation. That I do not think is going to happen. -- Yogendra
It is good to hear that an anti-invisibility cloack has also already been invented. We can move on now.
Huanyang (Kenyon) Chen has provided the following additional comments on his work on the anti-cloak: "The invisibility cloak hides any information in the outside world from the observers inside. So an object coated with the cloak is a “blind object� as it cannot see the outside. The observer inside the invisibility cloak can choose to see the outside world by putting a layer of anti-cloak material in contact with the invisibility cloak. In this way, the observer inside has some control of the situation. It is also very interesting to study whether there is a device that can reveal some items within a shield while leaving others hidden."